Deep ultra-hot melting residues as cradles of mantle diamond

The ancient stable continents are up to 250 km deep with roots extending into the diamond stability field. These cratons owe their mechanical strength to being cool and rigid, features inherited from extensive melt extraction. The most prominent model for craton formation envisages dominant melting at relatively shallow depth (50-100 km) above diamond stability followed by later imbrication to form the deeper roots. Here we present results from the first thermodynamic and geochemical modelling of melting at sufficiently high temperatures to produce the very magnesian olivine of cratonic roots. The new closed and open-system modelling reproduces the observed cratonic mantle mineral compositions by deep (c.200 km) and very hot melting (≥1,800°C), obviating the need for shallow melting and stacking. The modelled highly magnesian liquids (komatiites) evolve to Al-enriched and Ti-depleted as observed in the greenstone belts at the fossil surface of cratons. The paucity of Ti-depleted komatiite implies that advanced closed-system isochemical melting (>1,825°C) was much less common than open-system interaction between deeper liquids and melting of existing refractory mantle. The highly refractory compositions of diamond inclusion minerals could imply preferential diamond growth in the more reducing parts of the cratonic root, depleted by ultra-hot melting in response to heat plumes from a deeper former boundary layer that vanished at the end of the Archaean.